One of the important technologies supporting the recent rapid technical progress is the development of information technology, such as the computer. It is not too much to say that the development of performance of the aforementioned information technology can be attained by the development of performance and integration of the CPU (central processing unit) of information engineering equipment, i.e., ULSI (ultra large scale integrated) device constituting CPU. As one of the methods for drastically developing the performance and integration of ULSI devices, a method has been practiced which comprises developing the horizontal integration of ULSI, i.e., finely dividing elements, while developing the vertical integration of ULSI, i.e., multi-level interconnection of ULSI.
The most important factor of the aforementioned multi level interconnection of ULSI is to secure the depth of focus (DOF) of optical lithography by which the wafer is exposed to light through a pattern for metal wiring such as inter-layer dielectrics and metal wiring. In other words, it is required that the difference in height between rise and indentation in roughened surface be smaller than the DOF of the exposing light for patterning. To this end, leveling must be made with a high precision. In the process for forming a multi level interconnection, when there is a difference in height between rise and indentation in the inter-layer dielectrics or metal wiring, it is made impossible to effect sufficient focusing or form a fine wiring structure.
It is difficult to attain a high precision leveling by conventional SOG (spin on glass) or etching. As a substitute for these methods there has been normally used super precision polishing such as CMP (chemical mechanical polishing). The leveling by CMP is carried out by using an abrasive (normally referred to as “slurry”, which will be used hereinafter) having a particulate material such as silica and alumina dispersed in admixture in an alkaline or acidic chemically-corrosive aqueous solution and an elastic polishing material (hereinafter referred to as “polishing pad”) against the surface of an object to be leveled such as semiconductor wafer.
As the aforementioned polishing pad, there has been used a non-woven fabric or an elastic foam of polyurethane or the like. Particularly preferred among these materials is foamed polyurethane, which discharges no lint during polishing and has a surface cell structure capable of retaining the slurry. This surface cell structure can be produced in a controlled environment. The polishing pad made of the polyurethane foam has been heretofore produced by a process which comprises preparing a large block polyurethane foam (hereinafter referred to as “cake”) by a chemical foaming method under material and molding conditions arranged such that predetermined physical properties such as hardness and elasticity are provided, slicing the cake into a sheet having a predetermined thickness, and then punching the sheet into a shape, such as a disk, that is suitable for polishing.
However, the polishing pad produced by chemical foaming method is disadvantageous in that the size and dispersion of cells which are formed inside and have a great effect on the retention of the slurry governing the flatness of the final product cannot be sufficiently controlled and thus become non-uniform, making it difficult to perform polishing precisely enough to attain sufficient flatness.
The cake is sliced at a separate cutting step to obtain a sheet-shaped product on which a polishing pad having a predetermined thickness is produced. Therefore, the final polishing pad product has a surface flatness depending on the precision of the cutting step. However, since this precision is not sufficiently high, the precision in leveling of the object to be leveled is decided at the stage of selecting the material of the polishing pad. It is known that when a polymer material such as polyurethane foam is subjected to slicing or cutting, the viscoelasticity of the polymer material normally causes dislocation of blade cutting margin, i.e., so-called relief at the cutting portion, making it difficult to perform control to a precision of not greater than 10 μm.
On the other hand, improving polishing conditions (low pressure and high relative velocity polishing) or enhancement of hardness of the polishing pad can improve the leveling in the CMP. However, it is difficult to attain the former means due to the change of physical properties, i.e., viscoelasticity of polishing pad during polishing. The latter means is disadvantageous in that it is difficult to effect the predetermination of enhancement of hardness. Thus, it is apprehended that when the hardness thus attained is too high, the surface of the object to be polished can be scratched (damaged) by the polishing pad. The enhancement of hardness can be somewhat attained also by the enhancement of density. However, taking into account the retention of slurry on the surface of the polishing pad (capacity of rendering the amount of the slurry to be supplied into the surface of the wafer constant), a preparation method which cannot form fine cells, such as ordinary chemical foaming method and physical foaming method, encounters difficulty in cell distribution in proportion to enhancement of density, making it impossible to attain uniform distribution of cells. Further, as one of the aforementioned improvements in polishing conditions there has recently been proposed the use of a linear polishing type polishing pad having a long perimeter. However, this approach requires investment of funds for large-scale facilities, such as new polishing machines, and thus cannot be easily introduced, though being possibly a basic solution to these problems.
For CMP utilized particularly for isolation called shallow trench isolation (STI), a low pressure and high relative velocity polishing with a high hardness pad has been proposed as a device for lessening the dependence on isolation pattern. However, this approach is disadvantageous in that a polishing pad made of a polyurethane foam, if used, is subject to instantaneous rise of surface temperature due to friction with the surface of the object to be polished during polishing. As a result, the viscoelasticity of the surface of the polishing pad changes, rendering the removal rate, i.e., amount polished per unit time instable.